This morning on Public Radio's Performance Today, I heard about Mozart going back and forth with [another composer] with colorful sketches, markings, and quips within his original manuscripts like a couple 4th graders would. I was inspired and intrigued to find out more and see for myself.
Turns out you can peruse a digital archive of Mozart's and LOADS of other artists' original manuscripts and artwork on-line! By default, you are led to a crafty Shockwave viewer that lets you zoom, pan, and flip pages, but there is also an accessible version that might be easier to digest (and faster to dive into!).